4.7.0-rc7+: Oops during boot with USB pen drive inserted

[Chris Clayton <chris2553@googlemail.com>]

Hi,

With Linus' latest and greatest, I get an opps when I boot my laptop with a pen drive inserted in any USB port. The oops
message is:

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(8,2)

The oops seems to be 100% repeatable. If a USB pen drive is not inserted, the laptop boots successfully.

I've taken a photograph of the oops and you can view it at
http://s714.photobucket.com/user/chris2553/media/IMG_20160722_053841.jpg.html.

At the top of the picture, I notice that the partitions on my actual boot disk are being reported as being on /dev/sdb,
so it seems likely that, at this point, the pen drive is being seen as /dev/sda, although that has scrolled off the
screen. I don't boot via a ramdisk - my kernel has ext4 built in.  The grub2 entry is:

menuentry "Krisux, Linux 4.7.0-rc7+" {
    insmod ext2
    set root=(hd0,2)

    linux   /boot/vmlinuz-4.7.0-rc7+ ro root=/dev/sda2 resume=/dev/sda6 rootfstype=ext4 net.ifnames=0

}

(BTW, Krisux is not a real distro - it's just the name I have given Linux from Scratch system.)

The stack that is dumped is:

	dump_stack
	panic
	printk
	mount_block_root
	prepare_namespace
	kernel_init_freeable
	kernel_init
	ret_from_fork
	rest_init

I realise I could work around this by specifying the boot partition by, say, its UUID, but I thought you would want me
to report this anyway.

Of course, I'm happy to provide any other information required and to test any fix, but I will be out and about for the
next 14-16 hours, so it will be later tonight or maybe even tomorrow before I can respond.

Chris